Heavy metal removal is expected to be very similar to that of phosphorus removal although limited data are available on actual removal mechanisms. The removal mechanisms include adsorption, sedimentation, chemical precipitation, and plant uptake. One of the processes that assist in metals removal is burial as metal sulfide precipitates. The process is illustrated in Figure 6.2 (USEPA, 1999). One metal of concern is mercury. Under anaerobic conditions, mercuric ions are biometh-ylated by microorganisms to methyl mercury, which is the more toxic form of mercury (Kadlec and Knight, 1996). A process that may counteract the methy-lation is precipitation with sulfides, as illustrated in Figure 6.2. At Sacramento County, California, the mercury concentrations were reduced by 64% to 4 ng/L (Crites, et al., 1997). Metals removal depends on detention time, influent metal concentrations, and metal speciation. Removal data for heavy metals in the Sacramento County demonstration wetlands; in Brookhaven, New York; and in Prague are presented in Table 6.5. The removal of aluminum, zinc, copper, and manganese with distance down a Prague wetland is shown in Table 6.6.
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